Hey! Don't regret at all. I hate when people ask after they've personally dumped cash into a setup they are excited about, but they never did any research before they bought, then want opinions after.
If you aren't in yet, then you are asking now, before you are in, and you have a lot of time to make some considerations and let others think and respond. Are they planning on leaving that home theater setup as it is, or are they taking it with them?
Obviously, if they take it, then you are starting from scratch, in which case, you have a lot of questions to ask, and answer.
The first, and really most important (IMO) is establishing a budget for what you intend to do.
I would plan on painting. Keeping things dark as is already established, but make sure you go with flat paints. It looks like you could throw a fixed frame screen up for not a lot of cash. Something like a 120" diagonal or so, which can be decided upon later. Same with the projector really. You don't need to decide upon that until you know where cabling is, and what's there already.
I would focus on what you just asked about. Getting acoustics under control, and that question may be better served in the speakers and audio sections rather than the projector sections. Show the photo, let them know that you are buying the home and that you intend to rework the room as appropriate.
From there, you should get some good ideas on how to get the best audio performance from a 5.1 or 7.1 audio system. I would certainly make sure all wiring is hidden as best as possible, I would pick an equipment location that is OUTSIDE the room. The room is small, so having all your gear inside the room isn't the most convenient if it can be avoided, or in a small rack on the opposite end of the room from the screen. Covered so light doesn't spray onto the screen.
There are a number of ways to soften walls, but I haven't been in a room like that with concrete/brick surfaces everywhere. It's nice looking (IMO), but definitely appears cramped, and I wouldn't think the acoustics are good. It would likely get to sounding bright (shrill) very quickly. So, there are wall treatments which can be used. Nothing is really required for the ceiling, but there is stuff out there. I always use carpet in my theaters as a major absorber. Which is a shame with the nice hardwood, but perhaps a very large area rug with good padding would help.
If you aren't in yet, and you are buying this place with an intent of long term ownership, then just use this time to come up with ideas to make it all happen and be as awesome as you want it to be.